Low boiler pressure and leaking overflow pipe?

When the heating is on check the pressure if it rises a lot then the expansion vessel needs re-pressurising or replacing or the pipe connecting it to the boiler is blocked. If the pressure rises when the boiler is at rest then either the filling loop tap is not shutting correctly or the hot water heat exchanger is faulty. If none of these are happening then the safety valve is either faulty or it has dirt on the seat of the valve. Sometimes you can clear the dirt by switching off the electric boiler (purely to stop it coming on) follow the overflow pipe back to the boiler and you should see a br fitting with either a red or black plastic knob. Twist the knob (it will be stiff) until you hear the valve clunk, do that a couple of times re-pressurise the system (1-1.5 bar max) and check the overflow again. Just a few things to check, hope this helps.